All radio frequency transmitters employ amplifiers that boost the power of a desired signal from a relatively low level to a higher level. The output signals of the amplifiers are generally fed to an antenna system that broadcasts the RF signals to one or more remotely located receivers. For very high-power applications, radio frequency amplifiers still use vacuum tubes as the active elements for boosting the power of the radio frequency signals. However, for lower power applications, most amplifiers use solid state devices such as field effect transistors (FETs).
A common RF amplifier design uses only a single power FET in its output stage so that the amplifier is stable, i.e., free from oscillation, over a wide range of frequencies. However, amplifiers using only a single power FET are limited in the power levels they can produce. In order to increase the power rating of the amplifier, some RF amplifiers use two or more power FETs that are connected in parallel. Because each FET contributes only a portion of the total power, the combined power can be increased without damaging the individual devices. Such parallel high-power RF FET amplifier circuits are therefore generally more reliable than single transistor designs.
Thus, it is important to be able combine separate power sources. One well known prior art method is the Wilkinson combiner. Another prior art combiner is shown in U.S. Pat. No. 5,455,546 to Frederick et al. and assigned to the same assignee herein. Another improvement of a power combiner is disclosed in co-pending U.S. patent application Ser. No. 08/601,370 filed Feb. 14, 1996 entitled "High-Power Amplifier Using Parallel Transistors", assigned to the same assignee herein and hereby incorporated by reference in its entirety. In each of these combiners, it is important that the parallel power inputs be matched in phase.
One prior art method of constructing an RF phase detector is to use a double balanced mixer (DBM). Since each input is the exact same frequency, the output from the DBM will be a DC voltage proportional to the phase difference. The cost of a DBM in some applications can be relatively high. Thus, there is a need for a low cost phase detector.